The path integral Monte Carlo method (PIMC) provides a
nonperturbative, basis-set-independent, and fully correlated
calculation for quantum many-body systems at both zero and finite
temperature.[1]}, [2]}, [3]} However for
many-fermion systems, PIMC suffers from uncontrollable errors
arising from the notorious sign problem,[3]}, [5]}
which limits the accuracy or stability of the method. The origin of
the sign problem comes from the fact that the density matrix
can be positive or negative by even or odd permutations. At low
temperatures, contributions from positive and negative parts of the
density matrix almost perfectly cancel each other so that there is
no hope of extracting any useful information.
